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Author Topic: A Shocking Experience  (Read 4860 times)

Chuck Simon

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Re: A Shocking Experience
« Reply #10 on: August 29, 2014, 06:03:53 pm »

Can you send a good close-up picture of the DI box with the black mark on the corner? Al Keltz from Whirlwind watches this forum and I'm sure he'll get a big kick out of it.

As you can see, damage was minimal:


The inside looks good:


Is there anything I can test or check for?  It seems to be OK.
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Mike Sokol

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Re: A Shocking Experience
« Reply #11 on: August 29, 2014, 07:13:03 pm »

Is there anything I can test or check for?  It seems to be OK.

You should do a resistance test between pin-1 of each XLR and the phone jack ground on the opposite side of the box. You'll need to stick a metal phone plug in the jack for a test point.  It should read close to infinity (many millions of ohms) when that channel is in "Lift" mode, and close to zero (way less than 1 ohm) when that channel is in "Ground" mode. If both channels check out, then you should be good to go. If it always reads "open" (millions of ohms) not matter how you flip the "Lift/GND" switch, then you've burned up an internal ground path, which is what I would expect. In that case, consult with Al Keltz from Whirlwind on how to proceed.

Tough box...
« Last Edit: August 29, 2014, 07:19:45 pm by Mike Sokol »
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Chuck Simon

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Re: A Shocking Experience
« Reply #12 on: August 30, 2014, 10:11:31 am »

Well, it failed the test.  No signal passing through the ground on channel one.   I will be contacting Al.
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Al Keltz

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Re: A Shocking Experience
« Reply #13 on: September 04, 2014, 01:36:48 pm »

Just as we thought, ground trace was completely blown off the PCB. We replaced it with a piece of insulated wire. Transformers and all else are good to go and we'll be shipping it back today.

Did you find that the bass amp's safety ground was defeated or was it plugged into a power strip without the safety ground? Might be good to let him know. Things turned out OK but that practice could kill someone next time.

- Al
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Jerome Malsack

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Re: A Shocking Experience
« Reply #14 on: September 04, 2014, 01:52:50 pm »

May want to check the mixer grounds and the snake ?? 
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Al Keltz

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Re: A Shocking Experience
« Reply #15 on: September 04, 2014, 02:36:56 pm »

May want to check the mixer grounds and the snake ??

Good idea.

IIRC, Moderator Mike burned out an audio ground on a Mackie powered speaker just by demonstrating ground loop currents in one of his Frankenstein demos! there was a LOT more current through this DI, snake and mixer.
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John Roberts {JR}

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Re: A Shocking Experience
« Reply #16 on: September 04, 2014, 02:40:23 pm »

Back when I was designing products for the install market there was an UL ground bond requirement where external connections labelled ground had to sink tens of amps to mains ground with single digit voltage rise, to prevent voltage hazard on grounds while sinking the fault current until the circuit breaker opens. The ground opening up was a safety hazard, and I had one PC trace vaporize when tested.

For human safety around back line a more robust ground might be useful but not required. That old install product that failed ground bonding could have passed if i merely renamed the "ground" terminal something other than ground, but i went back and beefed up the ground trace on the PCB to not disappear when stressed by a mains fault.

JR

PS: Many audio connections to ground are not specifically denoted as safety grounds so not tested for integrity under high current faults. 
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Mike Sokol

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Re: A Shocking Experience
« Reply #17 on: September 04, 2014, 07:50:21 pm »

Good idea.

IIRC, Moderator Mike burned out an audio ground on a Mackie powered speaker just by demonstrating ground loop currents in one of his Frankenstein demos! there was a LOT more current through this DI, snake and mixer.

You are correct. I was intentionally putting around 3 amps ground-loop current through the shield, which is pin-1 on the XLR. This was to simulate what happens to a powered speaker connected to a mixer with a 3 volts ground voltage differential between the outlets. After about 5 or 6 seconds, the current dropped to zero and the hum stopped. An autopsy showed I had burned through the pin-1 trace on the powered speaker's circuit board. You were unknowingly doing this same experiment with a 120-volt differential between the two audio grounds. Ouch!  :o  So I'm thinking at least dozens of amperes flowing through that circuit board trace until it burned up. While the audio transformer in a WW DI can take a 120-volt common-mode differential (yes, I've tried this), that leaves you meat puppets as a potential ground fault current path. You have maybe 1,000 ohms internal resistance hand-to-hand or hand-to-foot, so that translates into roughly 120 mA of AC current through you if your hands and feet are wet. At 20 mA current you can't let go of a "hot wire", and just a few seconds of sustained 30 mA will probably put your heart into ventricular fibrillation. After that happens, and without CPR and medical care, it's just 5 short minutes to brain death and a dirt nap.  :'(

But back to your permanent pin-1 lift. Most of the time you might not even be aware that this had happened if you're always connecting in line-level input mode since it's essentially a permanent pin-1 lift. But if you plug in a condenser mic that needs phantom power, the mic will fail to operate, and the singer could get a 48-volt DC shock to the lips. Certainly it would be current limited to perhaps 10 mA DC and not really life threatening, but I had that exact scenario happen to me on a gig many moons ago which caused the singer to walk off stage because they had been shocked by the mic. I found an XLR cable in the signal path with shield/pin-1 connection broken. Silly me...  :o
« Last Edit: September 04, 2014, 08:15:28 pm by Mike Sokol »
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Chuck Simon

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Re: A Shocking Experience
« Reply #18 on: September 05, 2014, 04:45:50 pm »

Just as we thought, ground trace was completely blown off the PCB. We replaced it with a piece of insulated wire. Transformers and all else are good to go and we'll be shipping it back today.

Did you find that the bass amp's safety ground was defeated or was it plugged into a power strip without the safety ground? Might be good to let him know. Things turned out OK but that practice could kill someone next time.

- Al

Thanks.  The bass was connectected to AC power that was grounded - no strip.  His AC chord on the amp was damaged and hot and ground were shorted as far as I could tell.  He repaired it and the gig went on, but miced.  I was not going to take a chance with a direct box on that amp again.  No snake, the DI was connected directly to my Studiolive, which seemed to suffer no damage.  The DI saved it.

Thanks for the quick response, Al.  You have confirmed my belief that it pays to buy the best from a good company!
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TJ (Tom) Cornish

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Re: A Shocking Experience
« Reply #19 on: September 05, 2014, 04:54:52 pm »

Thanks.  The bass was connectected to AC power that was grounded - no strip.  His AC chord on the amp was damaged and hot and ground were shorted as far as I could tell.  He repaired it and the gig went on, but miced.  I was not going to take a chance with a direct box on that amp again.  No snake, the DI was connected directly to my Studiolive, which seemed to suffer no damage.  The DI saved it.

Thanks for the quick response, Al.  You have confirmed my belief that it pays to buy the best from a good company!
If the hot and ground were truly shorted at the amp, that must mean somewhere upstream of the amp the ground was disconnected, otherwise the amp would have blown the breaker instead of your DI.
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Re: A Shocking Experience
« Reply #19 on: September 05, 2014, 04:54:52 pm »


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